Engine mounted electronic control units for automotive engines are subject to a high level of heat that can limit or impair their performance. Typically, in order to reduce the high level of heat and enhance thermal performance, a printed circuit board that includes control circuitry is affixed to a cooling plate. For example, in a direct engine mount application, a printed circuit board is affixed to an aluminum substrate. The substrate provides mechanical support for the printed circuit board and assists in the dissipation of heat generated by components on the printed circuit board, which heat is conductively transferred from the components to the underlying aluminum substrate.
In one prior art method, printed circuit boards are affixed to a substrate by use of a pressure sensitive adhesive (PSA) tape. When manufactured, the adhesive tape, or film, is encased in liners that protect the film during shipment and storage. The adhesive film is then punched by a converter and manually cut into appropriately sized sheets. The sheets are manually loaded into a lamination machine along with the printed circuit boards and substrates. A release liner must be manually peeled off of the film sheets. The film is then applied to the substrate and a printed circuit board is applied to the adhesive film, thereby securing the printed circuit board to the substrate.
Use of adhesive film poses several problems. It is a manual process when manufacturing operations are becoming increasingly automated. Mistakes made in the manual handling of the film results in film that must be scrapped. Use of an adhesive film in the assembly of electronic control units requires a batch-oven bake cycle to prevent blisters from forming in the film during solder reflow, which baking process is lengthy, as long as twelve hours, and is disruptive of a high volume manufacturing process. In addition, acrylic adhesive films suffer from high temperature instability, typically at temperatures well below maximum operating temperatures of high power field effect transistors (FETs) and the reflow temperatures of lead-free solders, thereby imposing constraints on the manufacturing process and on the operation of electronic control units that employ the adhesive films.
In another prior art method, printed circuit boards are affixed to a substrate by use of a B-stage epoxy adhesive. A release liner sheet can be placed on the semi-cured adhesive to protect the adhesive for shipment and storage. Before lamination the release liner must be manually peeled off of the adhesive. The lamination machine then secures the printed circuit board to the substrate.
Use of a B-stage epoxy adhesive poses several problems. Mistakes made in the handling of the adhesive and the release liner film results in product that must be scrapped. Moreover, the epoxy has outgassing problems and is flammable. In addition, the epoxy requires two curing cycles which adds to processing.
What is needed is a technique to be able to use a dispensed liquid adhesive without causing significant damage to the adhesive. It would also be of benefit if the technique could utilize low-cost release liners, such as common release liners or even plastic films, which can be easily peeled off without significantly affecting the adhesive bonding surface property of the adhesive.